Insider Build Outputting 120Hz, While Reporting It Is Unsupported

from the xbox insiders reddit:

"

Xbox One Insiders - 120Hz FAQ

Q: I don’t see the dropdown under the Video Output settings that allows me to select 120Hz refresh rate.

A: The Refresh setting will only appear if your console and display support 120Hz over HDMI. 120Hz is only supported on Xbox One S and Xbox One X consoles, and while using a TV or monitor that supports 120Hz over HDMI. While some monitors support 120Hz over
DisplayPort, they may not support it over HDMI.

Please note that there is a bug in the 430 build that 120hz does not show if you meet these conditions but have the color depth set to anything BUT 8-bit. The new build today to Alpha (504 build) fixes this bug."

While the Series S is capable of 120hz output, almost no games on it support it. This is the budget system. It's not made for power.

If you wish this feature you're better off purchasing Series X

The X supports Arc however if your in preview there is a bug with it right now. Is your tv set to 120hz with 36 bits per pixel.? If it is disable 120hz and set Video fidelity to30 bits or lower. The insiders team are giving this has a work around for now.
You should also report has a bug via the insiders app.

1. Navigate to the Settings app on your Xbox One.
2. Choose the Display & sound tab
3. Select the Video output tile.
4. Open the Refresh rate drop-down menu.
5. Choose 120Hz to increase your refresh rate. (Your console should now output a 120Hz signal.)

That's the steps in the link. It doesnt give me the option for 120hz

A few things to consider....

Are you using an insider build of the xbox operating system?

And what colour depth have you selected?

Also, although monitor may support 120hz, it may only support it via display port and not hdmi.